An optical isolator is a non-reciprocal optical device that permits the transmission of light in one direction but blocks the unwanted reflection of light and the unwanted transmission of light. Due to this function, the optical isolator is attracting a lot of attention for use in high-power optical fiber lasers, optical amplifiers and high-speed optical fiber communication systems.
When linearly polarized light passes through a magneto-optical medium under a magnetic field, it rotates its plane of polarization by 45° and returns to the magneto-optical medium. The polarization plane of the back-reflected light rotates again by 45° so that the back-reflected light that rotated by 90° will be blocked and isolated.
The most common isolators are optical isolators that need bulk optical devices such as birefringent plates and special launching lenses, which need precision alignment and careful handling. In recent years, all-fiber optical isolators have attracted a lot of attention for use as all-optical devices in optical laser systems and fiber optical amplifiers, because they offer advantages including low insertion loss, high reflection properties and high isolation. Particularly, a high-power Yb-doped optical fiber laser (pulse type) system that operates at 660 nm (multi-kW power level) was developed for use in industrial and biomedical applications.
However, an all-fiber optical isolator that operates at a wavelength of 660 nm has not yet been developed. This is mainly because of the low isolating sensitivity of silica glass fiber, and this low sensitivity is due to the low magneto-optical sensitivity of the silica fiber at visible wavelengths (Verdet constant: −0.64 rad/Tm at 1550 nm, and −0.22 rad/Tm at 1310 nm).
Specialty optical fibers such as annealed fiber, twisted fiber and flint glass fiber have been proposed to reduce the linear birefringence of optical fiber thereby increase the magneto-optical sensitivity, but a complex fabrication process, a high slicing loss and high costs still remain as problems.
In an attempt to overcome these problems, phosphate or borosilicate glass optical fibers incorporated with Tb ions, which have a high Verdet constant, were reported, in which a rod-in-tube technique is used.
However, it was found that the propagation loss of the fibers is much larger than the propagation loss of silica optical fiber.